Bloody ethnic violence between Senegalese and Mauritanians in the Senegal River valley (the border region between Senegal and Mauritania) erupted in mid-April 1989, following the killing of two Senegalese peasants apparently by Mauritanian border guards, in a simmering dispute over animal grazing rights. Fighting soon forced the white (bidan) Moors who controlled trade in the region, to flee north; their rivals, the Poular-speaking blacks (Tukulors and Fulani), fled to safety on the south side of the river valley and made retaliatory raids into Mauritanian territory. Much looting and bloodshed by both sides occurred while at least 250,000 persons fled their homes into forces exile elsewhere. Attempts by the Organization of African Unity (OAU) to mediate failed in 1990. Senegal's President Abdou Diouf (1935-) managed to work out a prudent agreement which was signed by the two countries on July 18, 1991. The border was later reopened, and refugees and exiles began to return to their homes in the valley.